Content Pipeline tool cannot open Content.mgcb files

When I build and run a project, it works well, but when I want to add content to my game, the pipeline tool says “The document Content.mgcb could not be opened. Pipeline.MacOS cannot open files of this type” when I double click on it. What can I do to fix this? By the way, it says the version of the pipeline is 3.3 if that is helpful.

You need to start the pipeline separately (CMD+Space -> pipeline, or look for it inside your Applications folder), then click on File -> Open and from there navigate to the .mgcb of your project. A bit inconvenient perhaps, but double clicking the .mgcb is currently broken on MacOS for some reason.
You may also have to resize the pipeline window once to get its contents to show up. This seems to be a purely visual bug.